Phase interpolators are typical components in clock recovery circuits and related applications. To provide an interpolated clock signal, a phase interpolator interpolates between two or more input clock signals. A conventional phase interpolator interpolates to provide an output clock that ranges in phase between the two input clocks responsive to a tuning signal. The resolution of a phase interpolator between its input clocks depends on the resolution (number of unique states) of the tuning signal. For example, suppose that a phase interpolator interpolates between a 90 degree input clock and a 180 degree input clock. If the resolution of the tuning signal is 5 bits (32 levels), then there would be 30 intermediate phase values between the two extremes of 90 degrees and 180 degrees. Similarly, if the resolution of the tuning signal is 3 bits (8 levels), then there would be six intermediate phase values between the two extremes of 90 degrees and 180 degrees.
It is conventional for the tuning signal to be thermometer encoded. Each bit of the resulting thermometer-encoded tuning signal controls a corresponding slice in the phase interpolator. In particular, each bit controls whether a transistor conducts or not in the corresponding slice. For a 3-bit resolution, the phase interpolator would then include seven slices. The resulting thermometer-encoded 3-bit tuning signal would range from a value of [0000000] to [1111111]. For the tuning signal of all zeros, none of the corresponding transistors in the slices would conduct. For the tuning signal of [1111111], all of the corresponding transistors in the slices would conduct. Although the resulting phase interpolation is conventional, note that increasing the resolution requires additional slices. For example, 4 bits of resolution would require 15 slices whereas 5 bits of resolution requires 31 slices. The resulting multitude of slices consumes excessive die area and results in undesirable power consumption.
Accordingly, there is a need in the art for improved phase interpolators that can achieve greater resolution while conserving die space and power.